1. Field of the Invention
This invention relates to electrography, and to an improved dry electrographic developer composition which is useful in the development of electrostatic charge patterns. More particularly, the invention relates to a method for preparing an artifically aged or preconditioned dry electrographic developer having desirable characteristics which continue uniformly from the first prints through many thousands of prints. Previously available developers only attained such desirable characteristics after they had been aged in service, or if attained at the beginning of service, had tended to deteriorate as the number of prints increased. Developer produced by our novel method immediately provides prints which have image sharpness and image density which are superior to those produced with many previously available developers when first placed in service, which are equivalent in quality to those produced by many previously available developers after they have aged in service, e.g. after the preparation of several thousand prints, and which continue uniformly for many thousands of prints.
2. The Prior Art
Electrographic imaging and developing processes, and techniques, have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776 issued Nov. 19, 1940; 2,277,013 issued Mar. 17, 1942; 2,297,691 issued Oct. 6, 1942, 2,357,809 issued Sept. 12, 1944; 2,551,582 issued May 8, 1951; 2,825,814 issued Mar. 4, 1958; 2,833,648 issued May 6, 1958; 3,220,324 issued Nov. 30, 1965; 3,220,831 issued Nov. 30, 1965; 3,220,833 issued Nov. 30, 1965.
Generally these processes have in common the steps of forming an electrostatic charge pattern on an electrically insulating electrographic element, The electrostatic charge pattern is then rendered visible by a development step in which the charged surface of the electrographic element is brought into contact with a suitable developer mix. Conventional dry developer mixes include thermoplastic resin particles, known as toner particles, which may contain coloring agents, and may also include a carrier that can be either a magnetic material such as iron filings, powdered iron or iron oxide, or a triboelectrically chargeable, non-magnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium fluoride. The toner typically comprises a resinous material, a colorant like dyestuffs or pigments such as carbon black, and may also contain other addenda such as plasticizers, charge control agents and the like.
One method for applying a suitable dry developer mix to a charged pattern-bearing electrographic element is by the magnetic brush process. Such a process generally utilizes an apparatus of the type described, for example, in U.S. Pat. No. 3,003,462 issued Oct. 10, 1961, which customarily comprises a non-magnetic rotatably mounted cylinder having fixed magnetic means mounted inside. The cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, particles arrange themselves in bristle-like formations resembling a brush. The brush formations that are formed by the developer mix tend to conform to the lines of magnetic flux, lying substantially flat in the vicinity of the poles, and standing erect when said mix is outside the environment of the magnetic poles. Within one revolution, the continually rotating cylinder picks up developer mix from a supply source and returns part or all of this material to this supply source. This mode of operation assures that fresh mix is always available to the surface of the charged electrographic element at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer mix pickup, brush formation, brush contact with the electrographic element, e.g. a photoconductive element, brush collapse, and finally developer mix release.
In magnetic brush development, as well as in various other types of electrographic development wherein a two-component dry triboelectric mixture of a particulate carrier and a toner powder are utilized, e.g., cascade development such as described in U.S. Pat. Nos. 2,638,416 and 2,618,552, it is advantageous to modify the surface properties of the toner powder so that a uniform, stable net electrical charge may be imparted to the toner powder by the particulate carrier.
One method of developer preparation has involved placing particles of a carrier and particles of toner (containing a charge control agent in the concentration desired in the final developer, generally about 0.1 to about 6 parts by weight per 100 parts of resin) in a container such as a churn, crock, cylinder or barrel, and then rotating the container on its longitudinal axis for a mixing period which generally is 24 hours or less. Then the developer is placed in the developer station of an electrophotographic apparatus and the printing process begins. Generally the prints gradually improve in pattern sharpness until about 10,000 prints have been made. There may also be a decrease in pattern density for the first 1,000 to 5,000 prints, followed by a gradual and desirable increase through the next 20,000 to 30,000 prints, after which pattern density remains essentially constant at a desirable density.
Pattern density varies significantly with changes in relative humidity when a fresh developer is used, but sensitivity to relative humidity changes decreases as the developer ages, in particular, the pattern density at low relative humidity increases.
Certain observations have been made concerning the possible causes for variations in developer performance. A decrease in pattern density occurs with increasing toner electrical charge. Toner charge, in turn, increases with decreasing average toner particle size and decreases with increasing carrier scumming (the physical transfer of toner components to the surfaces of the carrier particles).
The average particle size of free toner increases during the initial stages of mixing with porous carrier particles because the fine particles of toner pack or fill the void spaces of carrier particles. Also, the average toner particle size decreases rapidly in the early life of the developer in printing apparatus, because the printed patterns seem to be formed by a selection of the larger toner particles. As printing proceeds, the average particle size of toner particles in the developer charge approaches a value which is smaller than that in fresh developer, and smaller than that in any replenishing charge of toner which is added periodically.
The response of a toner concentration monitor is also sensitive to toner particle size variations. As the average particle size decreases, the carrier particle surface covered per unit weight of toner increases. This appears to the monitor as an effective concentration increase since the reflectance of the developer decreases, and the actual toner concentration decreases,
Developer resistance changes occur as a result of a) toner particle size variations, b) attrition in carrier particle size by physical action during circulation in the developer system, and c) scumming of the carrier particles.
Toner throw-off is also related to toner charge level, particle size distribution, and changes in surface characteristics of the carrier particles.